A. Field of the Invention
The invention relates to the field of metal forming, and particularly to a method of making metallic sandwich structures incorporating filamentary reinforcements by superplastic forming and diffusion bonding.
B. Description of Prior Art
It has been known for many years that certain metals, such as titanium, and other alloys, exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with a reduced tendency toward necking. This capability is exhibited by only a limited number of metals and alloys, and within limited temperature and strain rate ranges. For example, some titanium alloys such as Ti-6Al-4V have been observed to exhibit superplastic characteristics.
Until the advent of viable superplastic forming techniques, taking advantage of this property to form complex configurations requiring large tensile elongations was extremely difficult or, in some instances, not possible. A significant breakthrough in superplastic forming was made by Hamilton, et al., disclosed in U.S. Pat. No. 3,934,441, "Controlled Environment Superplastic Forming", incorporated into this specification herewith by reference. Simplified, the process involves placing a worksheet or blank and a shaping member in a chamber preferably with an inert gas environment. The blank is heated to a temperature where it exhibits superplastic characteristics whereafter pressure is applied to the blank, causing it to stretch and form about the shaping member.
Diffusion bonding refers to the metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a sufficient time so as to cause co-mingling of the atoms at the joint interface. Diffusion bonding is accomplished entirely in the solid state at or above one-half the base metal melting point. Actual times, temperatures and pressures will vary from metal to metal.
The combining of superplastic forming and diffusion bonding (SPF/DB) in the making of metallic sandwich structures has been successfully accomplished and is disclosed in U.S. Pat. No. 3,927,817, "Method of Making Metallic Sandwich Structures," by Hamilton, et al., and is herewith also incorporated herein by reference.
Basically, the Hamilton, et al. method for making metallic sandwich structures involves fabricating the structures from a plurality of metal blank workpieces. One or more of the blanks are treated in the selected areas not to be diffusion bonded. The blanks are positioned in a stacked relationship and placed in a forming apparatus in the form of a die assembly. The stack is diffusion bonded together at the untreated areas and at least one of the blanks is superplastically formed against one or more of the die surfaces forming the sandwich structure. The core configuration is determined by the location, size, and shape of the joined areas.
While the Hamilton, et al. SPF/DB process has demonstrated cost and weight reductions, considerable competition has been received from structures made from advanced epoxy matrix composite materials because of their light weight and selected high strength and stiffness characteristics. Thus, the main application of SPF/DB fabricated parts has been in high temperature applications where the epoxy system is unuseable. The incorporation of the filamentary material also increases the operational temperature range for conventional titanium structures in that the filamentary material maintains its strength at temperatures that begin to weaken the titanium. Thus, if metal matrix technology could be applied to the SPF/DB process, the number of probable structures thus fabricated would be greatly enlarged.
Metal matrix composite materials are far from new; for example, U.S. Pat. No. 3,991,928, "Method of Fabricating Titanium Alloy Composite Materials," by L. A. Freidrich, et al. discloses such a method. Typically, silicon carbide or boron filaments are sandwiched between layers of titanium alloy sheets. The filaments are initially positioned on a matrix material by means of adhesive bonding; i.e., a suitable plastic such as polystyrene is employed to maintain the fibers in an evenly-spaced parallel orientation. The plastic material is selected such that it will evaporate during the consolidation step. Consolidation is accomplished by heating to a high temperature, and pressing under high pressures in the range of 5,000 to 25,000 pounds per square inch. Typically, the stacked material is sealed in a retort upon which a vacuum is drawn prior to pressing. Such material has extremely high modulus and strength to weight ratios, and thus are capable of producing a light and rigid structure.
U.S. Pat. No. 4,163,380, "Forming of Pre-consolidated Metal Matrix Composites," by Massner discloses a method of forming metal matrix composite material by placing the material between a pair of dies, heating and thereafter closing the dies at a rate of 5-10 mils per minute. Since, the composite material is not in the superplastic temperture range, forming pressures are exceeding high. Therefore, this process has the disadvantage of requiring large presses and expensive high strength dies.
Up until now, it was not thought possible to incorporate metal matrix composites into structures formed by the SPF/DB process because of the high strain rates produced; i.e., on the order of 1.times.10-4 inch/inch/second. Such strain rates if applied to metal matrix composites would cause breakage of the filaments. Breakage occurs because of the adherence of the metal matrix material to the filaments, which when superplastically formed, would cause very high tensile loads to be induced into the filaments.
Therefore, it is a primary object of this invention to provide a method of making lightweight structures having increased strength and stiffness.
Another object of this invention is to provide a method of making structures incorporating metal matrix composite materials by superplastic forming/diffusion bonding.
A still further object of this invention is to provide a method of making superplastically formed/diffusion bonded structures having selected strength reinforcement.